Obtaining and maintaining the isochronism of a mechanical timepiece oscillator are major difficulties of watch design. The quality of the balance spring is an essential factor of the precision of a timepiece. Numerous advances have been made, particularly by implementation of twisting inner or terminal coils.
Other research has made it possible to maintain a flat development of the balance spring. Thus, BE Patent No 526689 in the name of Emile MICHEL, or CH Patent No 327796 in the name of ASUAG propose a balance spring wherein at least one part of the strip has a straight section different from that of the rest of the strip, in order to obtain, during operation, a concentric development of the balance spring. However, the teaching is very general, and those skilled in the art have to perform lengthy experiments to define suitable parameters.
EP Patent Application No 2299336 A2 in the name of ROLEX SA discloses a flat balance spring for a timepiece balance including a wound coil, arranged to ensure a substantially concentric development of the balance spring, and almost zero force exerted on the pivots and the rigid point of support, during a rotation of less than 360° of the inner end relative to the outer end thereof in both directions from its rest position. The rigidity of the strip decreases gradually and over more than 360°, from each end thereof, and more specifically from, on the one hand, a point located between the inner end and second coil thereof, and on the other hand, from a point located between the outer end and penultimate coil thereof, the lowest rigidity being located in the median part of the strip.
EP Patent Application No 2151722 A1 in the name of ROLEX SA concerns a balance spring including a plurality of strips wound into a spiral, in one piece with a common collet of larger section than the sections of the coils of the various strips, and in one piece with an outer common ring of larger section than the sections of the coils of the various strips.